Process for making ferromagnetic metal powders

ABSTRACT

Disclosed herein is a process for making ferromagnetic powders from salts of iron, cobalt, nickel, and from 0 to 35 percent of a chromium salt, by total weight of the salts. The process comprises reacting the metal salt or salts with a reducing agent of (a) an amine-borane and (b) a tetrahydroborate.

United States Patent Little, Jr. et al.

[451 May 16, 1972 [54] PROCESS FOR MAKING F ERROMAGNETIC METAL POWDERS[72] Inventors: Ernest L. Little, Jr.; Jack D. Wolf, both of [21]App1.No.: 78,182

[52] US. Cl. ..l48/105, 75/0.5 AA, 75/108, 75/119,148/103,148/108 [51}Int. Cl ..H0lf l/06, HOlf l/ZO, C22b 23/04 [58] Field ofSearch..148/100, 103, 105,108,3155, 148/3157; 75/0.5 AA, 108, 119

[56] References Cited UNITED STATES PATENTS 3,206,338 9/1965 Miller etal.. ..148/105 3,369,886 2/1968 Metzger ..75/108 X 3,535,104 10/1970Little, Jr. et a1 ..75/0.5 AA 3,567,525 3/1971 Graham et a1. 148/105 XFOREIGN PATENTS OR APPLICATIONS 843,367 6/1970 Canada ..75/108 OTHERPUBLICATIONS R. Paul et al., Catalytic Activity of Nickel Bodies,Industrial and Engineering Chemistry, Vol. 44, No. 5, 1952), Pages 1006-1010.

Primary ExaminerL. Dewayne Rutledge Assistant Examiner-G. K. WhiteAttorney.1amcs A. Costello [57] ABSTRACT Disclosed herein is a processfor making ferromagnetic powders from salts of iron, cobalt, nickel, andfrom 0 to 35 percent of a chromium salt, by total weight of the salts.The process comprises reacting the metal salt or salts with a reducingagent of (a) an amine-borane and (b) a tetrahydroborate.

12 Claims, No Drawings PROCESS FOR MAKING FERROMAGNETIC METAL POWDERSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a new process for making ferromagnetic metal powders by thereduction of ferromagnetic metal ions.

2. Description of the Prior Art Amine-borane reducing agents are knownto reduce metal salts to form metal films on catalytic surfaces such asthe surfaces of objects to be plated. Metal powders, heretofore, couldnot be made using amine-borane reducing agents. Although it is known toreduce salts of magnetic metals to metal powders using tetrahydroboratessuch as sodium tetrahydroborate, the powders are often non-magnetic.See, for instance, Raymond Paul et al., Industrial and EngineeringChemistry, 44, 1006-1010, 1952).

SUMMARY OF THE INVENTION Salts containing the Bl-lfanion are referred toherein as tetrahydroborates. They are also known in the literature asborohydrides. The tetrahydroborate salts have now been found to promoteor trigger the reduction of ferromagnetic metal salts by amine-boranes.It is believed that this triggering effect is the in situ generation ofcatalyst sites. Subsequent reduction by amine-boranes takes place,selectively, at these sites to form homogeneous ferromagnetic metalpowders.

The novel process comprises reacting, in solution, a salt or salts of atleast one of the metals selected from the group consisting of iron,cobalt and nickel, and from to 35 percent, by total weight of the salts,of a chromium salt, with a reducing agent comprising (a) an amine-boraneand (b) a tetrahydroborate; wherein the molar ratio of the amineboraneto the tetrahydroborate is from about 3 to l to 700 to 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A wide variety of iron, cobalt,nickel and chromium salts may be employed herein. Preferred salts aresoluble in water, and for this reason halides, sulfates, nitrates,fluoroborates, and acetates are customarily used. Ferrous salts arepreferred to ferric salts because less reductant is required. Chromoussalts oxidize rapidly in air, consequently it is preferred to usechromic salts.

The process of this invention produces ferromagnetic metal powders oflow boron content. The low boron content of these powders helps explaintheir excellent magnetic properties. See Tables I, II, III and IV inthis regard. Furthermore, the use of metallic salts of lower alkanoicacids favors formation of products of especially low boron content.Consequently, salts of lower alkanoic acids constitute a preferredclass. Within this class, formates, acetates (Ac), and propionates areespecially preferred because of their solubility in water. Other usefulmetal salts include ferrous sulfate, ferrous chloride, ferrousfluoroborate, ferrous propionate, cobalt sulfate, cobalt chloride,cobalt nitrate, cobalt fonnate, nickel chloride, nickel sulfate, nickelacetate, chromic sulfate, chromic chloride, and potassium chromicsulfate.

Any ratio of iron, cobalt and/or nickel salts can be used. The alloycomposition of the powder product depends on the particular metal saltsemployed. The relationship between metal salt composition and alloycomposition can be readily determined for any system. The amount ofchromium salt, if one is used, should be up to about 35 percent byweight of the mixture of salts.

In general, any amine-borane can be employed herein. In amine-boranes,the nitrogen of an organic amine is coordinately bonded to a EH moiety,for example, as in dimethylamine-borane, (CI-l NH'Bl-l The amine portionof the amine-borane can be a primary, secondary, or tertiary amine. Itcan be aliphatic, aromatic, or heterocyclic, or can have more than oneof these characteristics. It can be a monoamine, a diamine, or apolyamine. In amine-boranes formed from diamines and polyamines,normally each amine nitrogen will be coordinately bonded to a BI-lmoiety.

Examples of operable amine-boranes are methylamine-boraneethylamine-borane isopropylamine-borane t-butylamine-boranecyclohexylamine-borane aniline-borane p-toluidine-boranem-anisidine-borane p-chloroaniline-borane p-bromoaniline-boranediethylamine-borane morpholine-borane di-t-butylamine-boranediisopropylamine-borane dipropylamine-borane diisopentylamine-boranedibutylamine-borane piperidine-borane pyridine-borane N,N-dimethylcyclohexylamine-borane dimethyloctadecylamine-borane N ,N ,N,N'-tetramethylenediamine-bisborane triethylamine-boraneN-methylmorpholine-borane dimethylethylamine-boraneN,N-dibutylcyclohexylamine-borane diethylbutylamine-boraneN,N-diethylcyclohexylamine-borane N,N-dimethylaniline-boraneN,N-dimethyl-p-toluidine-borane Useful amine-boranes derived fromsecondary and tertiary amines are readily available and stable.Preferred are amineboranes containing aliphatic or heterocyclic amines,or amines having both these characteristics such asN-methylmorpholine-borane. Monoamine-boranes are also preferred.Especially preferred, because of their stability and solubility in thesolvents ordinarily used for the process, are amine-boranes of theformula R ,,NH,,-BH where the R's are the same or different, preferablythe same, and are lower alkyl, and n is 0 or 1. Of these,trimethylamine-borane dimethylamine-borane, and triethylamine-borane aremost preferred.

The volume, Boron, Metallic-boron Compounds, and Boranes, lnterscience,1964, R. M. Adams, is a general reference to the preparation of theamine-boranes and tetrahydroborates which are useful in the practice ofthis invention.

Contemplated tetrahydroborates include alkali-and alkaline-eanh-metaltetrahydroborates, tetraalkylammonium tetrahydroborates and alkali-metaltris (lower alkoxy) tetrahydroborates. Sodium and potassiumtetrahydroborates are preferred. Other useful tetrahydroborates arelithium tetrahydroborate, magnesium tetrahydroborate, calciumtetrahydroborate, tetramethylammonium tetrahydroborate,tetraethylammonium tetrahydroborate, tetraisopentylammonium,tetrahydroborate, ethyl tri-s-butylammonium tetrahydroborate,dihexyldimethylammonium tetrahydroborate, and potassiumtributoxytetrahydroborate.

The amine-borane/tetrahydroborate weight ratio can vary widely and canbe from about 5/1 to 1000/1. Preferably it is between about lO/l and600/1. 'Altematively, the amineborane/tetrahydroborate mole ratio canrange from about 3/1 to 700/ l and preferably is between about 6/1 and400/ l.

The ratio of amine-borane/metal salt is not critical; up to percentstoichiometric excess, or even greater, of either reactant can be used.It is preferred to use a slight stoichiometric excess, say, 10-50percent of metal salt over amineborane. In the stoichiometry of thereaction, it is assumed that the amine-borane has three equivalents ofreducing power per BI-l moiety. That is, each hydrogen bonded to boroncan reduce the oxidation number of a metal ion by l.

Although there is no necessity for a particular order or reactantaddition, it is preferred to add the tetrahydroborate to a mixture ofthe metal salts and amine-borane. Addition of the metal salt solution toa mixture of the reducing agents may result in less eflicient reduction.The addition of the amineborane to a mixture of the tetrahydroborate andmetal salts, after a time lapse of, say, 30 minutes or more, may alsoresult in loss of efficiency.

In most cases, much more powder product is produced than isstoichiometrically possible from the relatively small amounts oftetrahydroborate employed. However, iron-rich metal salts are especiallydifficult to reduce, by any reductant, and consequently it is necessaryto employ more than an equivalent amount of the tetrahydroborate toinitiate the formation of iron-rich alloys.

Although water is a convenient medium for carrying out the process ofthe invention, the metal salts, amine-boranes, and tetrahydroborates aresoluble in other media, particularly aqueous solutions of water-miscibleorganic liquids such as methanol, ethanol, acetone, tetrahydrofuran, l,Z-dimethoxyethane, 2-ethoxyethyl ether, propyl alcohol, and isopropylalcohol. Solvents of this sort may be substituted for water and in factmay be preferable in certain instances. An example of such an instanceis one in which the amine portion of the amine-borane has a relativelyhigh carbon content. The proportion of the organic solvent in mixedaqueous-organic media is primarily governed by its effect on thesolubility of components of the reaction mixtures; the organic solventusually will not exceed the volume of the water used.

To improve the magnetic properties of the powders produced, the novelprocess can be carried out in a magnetic field. This will generallyincrease coercivity and remanence by fostering chaining of theparticles. Reaction mixtures are generally agitated manually ormagnetically. It is important when the process is carried out in amagnetic field, to control agitation so as to enhance the formation ofchained particles. A rotating magnet can be used to promote mixing andto provide the desired magnetic field.

The process is most conveniently carried out at ordinary temperatures(2030 C.), although temperatures of up to about 45 60" C. can be used.Temperatures below 20 C. can be used, but no advantage results. Apractical low limit is imposed by the freezing point of the particularmixture being used. Pressure is not a critical variable. Atmosphericpressure is usually used for convenience, but higher or lower pressurescan be used.

Under ordinary conditions, the reaction begins immediately upon mixingthe reactants, as shown by precipitation of a dark solid product.Reaction is substantially complete within a few minutes, but the mixtureis usually allowed to stand for about 15 minutes to insure completereaction.

The product is isolated by filtration and washed with water and withacetone to remove by-products. Then, it may be allowed to stand inacetone for about to 24 hours before final isolation and drying. Agingin acetone eliminates any tendency of the particles to be pyrophoric.Alternatively, aging can be accomplished by drying the product in a flowof an inert gas such as argon, to which gradually increasingconcentrations of air (oxygen) are added.

The ferromagnetic powders made herein may contain oxygen in the form ofmetal oxides, hydroxides or adsorbed moisture. The presence of oxygen inone of these forms is helpful eliminating pyrophoric tendencies in thepowders. In most cases, the elimination of the pyrophoric tendencies bythe oxygen is accomplished without unduly affecting magnetic properties.All the powders are useful in the preparation of magnetic tapes andpermanent magnets.

SPECIFIC EMBODlMENTS The following Examples illustrate the invention andare included within the limits of the invention without setting thoselimits.

All magnetic properties reported in the Examples were determined bypacking the powders in tubes and placing the tubes in an extractionmagnetometer with an applied field of about 4,400 Oersteds (Oe). Theapparatus is similar to that described in T. R. Bardell on pp. 226-228of "Magnetic Materials in the Electrical Industry," PhilosophicalLibrary, New York (1955 Saturation magnetization 0-,, and remanentmagnetization, 0,, are given in the Examples as emu/g. Intrinsiccoercive force, il-lc, is expressed in Oersteds. Intrinsic coerciveforce is defined in Special Technical Publication No. of the AmericanSociety for Testing Materials, entitled "Symposium of Magnetic Testing"(1948), pp. 191-198. Values of intrinsic coercive force were determinedon a DC. ballistic-type apparatus which is a modified form of theapparatus described by David & Hartenheim in the Review of Scientificlnstruments 7, 147(1936). V 1 EXAMPLE 1 A solution of 0.5 g of sodiumtetrahydroborate in 50 ml of water was added to a solution of 56.2 g of(3080 -711 0 and 5.9 g of dimethylamine-borane in 400 ml of water. Theresulting mixture was manually stirred only enough to insure completemixing. After 15 minutes at room temperature, the reaction mixture wasfiltered. The dark solid on the filter was washed with water (500 ml),washed with acetone (500 ml), and suspended in 125 ml of acetoneovernight. It was then separated by filtration and air0dried, to give3.9 g of a ferromagnetic solid that contained 89.36 percent cobalt, 3.72percent boron, and 5.26 percent oxygen.

EXAMPLES 2-19 By the method of Example 1, a number of other reductionsof various cobalt salts by dimethylamine-borane in the TABLE I NBBHI,Magnetic Product, Percent Percent;

Ex Cobalt salt 5. field 3. Co 1H0 v 0. b0 3. 9 89. 4 3. 72 175 98 24 0.25 3. 6 92. 0 3. 47 18 0. 10 2. 4 91. 8 3. 40 65 111 19 0. 05 l. 4 91. 83. 64 75 106 14 0. 01 1. 4 93. 0 3. 16 35 118 8 0. 60 3. 1 89. 6 3. 24185 104 20 0. 26 2. 6 85. 0 3. 39 175 102 25 0.10 1. 0 71. 8 3. 39 90109 '20 0. 05 l. 0 90. 6 3. 32 75 65 13 0- do 0. 01 0. 6 90. 6 85 109 2011..." 50 g. of COAczAHzO 0. 60 4. 7 81.3 1. 82 55 121 12 I2.. Same asabove 0.10 4. 2 92. 7 0. 96 40 11 13 d 0.06 2.7 99.3 1.03 40 123 13 0.010. 5 88. 2 1. 08 50 13 0. 60 6.1 88. 8 1. 74 85 102 14 0. 05 1. 8 93.7 1. 14 60 130 16 do 0. 01 1. 4 93. 8 1.10 46 128 15 47.8 g. 0!(30011111110 0. 50 4. 5 90. 4 3. 20 104 22 Same as above 0. 50 3. 3 89.8 3. 61 220 102 26 EXAMPLES 20-25 The embodiments of the invention inwhich an exclusive Examples 20 to 25 represent experiments carried outby the property or pnvllege ls clam-led are defined as follows:

method of Example 1. In Examples 22 to 25 mixtures of meta] A for makingferromagnetic metal Powders salts were reduced. In each experiment themetal salt/- 5 pnsmg reactmg amineOborane solution was made up by addinga solution of Salts of at least one Ofthe metals iron, Cobalt, nickel,and 5.9 g of dimethylamine-borane in 200 ml of water to a solution from0 to 35 1361136"t of a chromium Salt, based t01111 of the metal salt orsalts in 200 ml of water. In none of the exwelght 9 the Salts amples wasthere any evidence of reduction of the metal salts wlth a redufimg agentof until the tetrahydroborate was added. The final products wereamme'borane and all black powders having magnetic properties. Theseexperiafietrahydroborate mems are Summarized in Table 1L wherein themolar ratio of the amine-borane to the TABLE II Product, Percent PercentEx. Metal salts g. metal B 1H 5. a,

50 g. NlAcz-4HzO 4. 6 93.1 Ni 3. 68 -30 13 5 21 5g 8 K $0284 $518. 0. 9go 1. 30 405 99 42 g. e g o 22.. g g sg' g g 2.1 C0 1.74 735 77 35 55. ge 4'7 42.9 Fe z3 {gO g g gyg 0.8 $28 1.73 435 3s 11 B- 10 ("2- 2 0 1L ,8g g g 5.7 02m 140 55 11s 10 5 R. lAcr'i z 1 94.1 NI .5 K cmcrzmu 5 o Cr3.41 50 11 2 EXAMPLES 26-34 tetrahydroborate is from about 3 to l to 700to l.

. 2. A rocess accordin to claim 1, wherein one of the metals The methodof Example I was used to reduce a series of isiron p g iron and n ckelsalt mixtures. In each experiment, a solution of A process according toclaim 1, wherein one of the metals 5.9 g of dimethylamine-borane in 200ml of water was added is cobalt to a Solunon of and mckel Salts 200 mlof water In a 4. A process according to claim. 1, wherein one of themetals vessel on the poles of a permanent lSOO-Oe magnet. There 30 isnickel. was no evldeflce of redlfcnon of the mefal over a P 5. A processaccording to claim 1, wherein one of the metals of about 5 minutes atthis stage. A solution of 0.5 g of sodium is chromium.

tetrahydroborate in 50 ml of water was then added, and the reaction wasinitiated and the process carried out as in Example l. The details ofthese experiments are summarized in Table III. All the products wereblack solids having magnetic properties. In this series the ratio Fe/Niin the product was tetrahydroborateis sodium tetrahydroborata similar tothe Fe/Ni ratio in the original salt mixture. As can be A processaccording to claim 1, wherein the seen from the Table, both a, and 0,.tend to decrease as nickel tetrahydroborate is potassiumtetrahydroborate is increased.

6. A process according to claim 1, wherein the salts are selected fromthe group consisting of halides, sulfates, nitrates, fluoroborates,formates, acetates and propionates.

7. A process according to claim 1, wherein the TABLE III FeSOflHzO,NiSOa-fiHzO, Product, Percent Percent Percent g. g. g. Fe Ni B .11., tr,a,

EXAMPLES 35-36 9. A process according to claim 1 carried out in amagnetic field.

10. A process according to claim 1, employing an amineborane of theformula,

By the general procedure of Example 1, tetrahydroborates other thansodium tetrahydroborate were used to promote the 55 reduction of cobaltion by dimethylamine-borane. In each experiment, the solution 0.5 of thetetrahydroborate in 50 ml of water was added to a solution of 56.2 g ofCoSO '7H O and 5.9 g of dimethylamine-borane in 400 ml of water in avessel placed on the poles of a permanent 1,500-Oe magnet. The when?!theR Sarelowei T g or h details of these experiments, which producedblack solids, are A process according to c alm w erem t eamulesummarized in Table borane is dimethylamineborane. I

12. A process according to claim 10, wherein the amine- TABLE IV boraneis dimethylamine borane and the tetrahydroborate is Tetrahydrosodiumtetrahydroborate and the mole ratio is between about Ex. borate ProductCo B iHc 0', 0', 6 to 1 to 400 to 1, respectively.

NBH4 36 KBI-l 3.l g. 90.4 3.25 165 98 22 t

2. A process according to claim 1, wherein one of the metals is iron. 3.A process according to claim 1, wherein one of the metals is cobalt. 4.A process according to claim 1, wherein one of the metals is nickel. 5.A process according to claim 1, wherein one of the metals is chromium.6. A process according to claim 1, wherein the salts are selected fromthe group consisting of halides, sulfates, nitrates, fluoroborates,formates, acetates and propionates.
 7. A process according to claim 1,wherein the tetrahydroborate is sodium tetrahydroborate.
 8. A processaccording to claim 1, wherein the tetrahydroborate is potassiumtetrahydroborate.
 9. A process according to claim 1 carried out in amagnetic field.
 10. A process according to claim 1, employing anamine-borane of the formula, R3 nNHn.BH3, wherein the R''s are loweralkyl and n is 0 or
 1. 11. A process according to claim 10, wherein theamine-borane is dimethylamine borane.
 12. A process according to claim10, wherein the amine-borane is dimethylamine borane and thetetrahydroborate is sodium tetrahydroborate and the mole ratio isbetween about 6 to 1 to 400 to 1, respectively.